get enum with default value

[IamRezaMousavi]

Description

I want to get enum value or use default one if it has error, but I can't

sample code:

#include <nlohmann/json.hpp>

#include <iostream>
#include <string>

enum class UserRequestType {
    GetUser,
    AddUser,
    Unknown
};

// clang-format off
NLOHMANN_JSON_SERIALIZE_ENUM(UserRequestType, {
  {UserRequestType::GetUser, "get"},
  {UserRequestType::AddUser, "add"},
  {UserRequestType::Unknown, "unknown"}
})
// clang-format on

int main(int argc, char const *argv[]) {
  std::string jtext = "{\"type\": \"invalidtype\"}";

  nlohmann::json j = nlohmann::json::parse(jtext);
  UserRequestType t = j.value("type", UserRequestType::Unknown);

  std::cout << static_cast<int>(t) << '\n'; // got 0 (UserRequestType::GetUser)
  return 0;
}

---

I know I can use j["type"].get<UserRequestType>() and reorder the enum to use first one as default, but json.value(string, UserRequestType has a bug

Reproduction steps

compile and run the code

Expected vs. actual results

expected j["type"].get<UserRequestType>() throw exception and j.value("type", UserRequestType::Unknown) return default one

Minimal code example

Error messages

Compiler and operating system

Ubuntu 24.04, GCC 13.3.0

Library version

3.11.3-1

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[gregmarr]

Working fine here: https://godbolt.org/z/n6jqdTPhf
Oh, I see, you want value() to override the default from NLOHMANN_JSON_SERIALIZE_ENUM

#4989 would give NLOHMANN_JSON_SERIALIZE_ENUM_STRICT which would throw in your case, and then you could use the default, but you'd need to do the catch and replace manually.

Yeah, your best bet is to reorder the NLOHMANN_JSON_SERIALIZE_ENUM elements to have unknown as the first, and then if the Json has a bad value, you'd get unknown as the value. You'd still want to keep Unknown in the value() call as that is used when "type" doesn't exist in the JSON.